The file association bug is actually present in 1803 as well, not just 1809. A cumulative update some months ago introduced the bug and subsequent updates never fixed it, and somehow it got integrated into the ISO for 1809 so you'll be fucked on a fresh install. A fresh install of 1803 will be fine... so long as you assign all extensions before updating to the latest build.

It's a complete clusterfuck and makes me wonder why I'm so lazy and incapable of moving to Linux.